Fluorescent lighting is a very common type of illumination. Fluorescent lamps function when an electrical arc is established between two electrodes located at opposite ends of the lamp. The electrical arc is established by supplying a proper voltage to the lamp. The lamp is filled with an ionizable gas and a very small amount of vaporized mercury. When the arc is established, collisions occur between the electrons and the mercury atoms, causing the emission of ultraviolet energy. The fluorescent lamps have a phosphorous coating on their inner surface, which transforms the ultraviolet energy into diffused, visible light. In order to establish the electrical arc, and thus turn on the lamp, a high voltage is typically required. However, once the lamp has been turned on, a lesser voltage is required to maintain the lamp's operation.
In order to start and operate a fluorescent lamp, a fluorescent lamp ballast is used. Among other functions (such as limiting the current flow through the lamp once it has already been started), a ballast is a device which provides the appropriate voltage to establish the arc through the lamps. Several different kinds of ballasts currently exist, e.g.--series mode and parallel mode. The series mode operates lamps in series across the output voltage of the ballast. The series mode ballast, while capable of performing dimming applications, usually is complex and thus, requires relatively high manufacturing cost. Parallel mode ballasts, while being less complex, and less expensive, are typically unsuitable for dimming applications, as will be explained below.
FIG. 1 shows a schematic diagram of a prior art parallel resonant current-fed circuit, coupled to a DC supply source 190, which functions in a fluorescent lighting ballast. Transformer 101 contains a first primary winding comprising windings 111 and 112 and second primary winding comprising windings 121 and 122. Additionally, the first primary windings of transformer 101 is connected in parallel with capacitors 161, 162 and 163. Primary windings 111 and 112, and capacitors 161, 162 and 163 form a tuned circuit, also known as an L-C parallel resonant circuit, and in conjunction with the other components of the circuit, produce an oscillating action upon the introduction of a start-up current.
Linear inductor 151, is coupled to a center tap terminal 105 of first primary winding of transformer 101 so as to provide a substantially constant current signal to the center tap terminal. Linear inductor 151 is also coupled to a drive terminal 102 of the second primary winding of transformer 101 through a resistor 141, so as to provide the start-up current feed to transistors 131 and 132 respectively. The current feed is sufficient to provide the minimum base drive current required by transistors 131 and 132 to start the transistors to operate in an oscillation mode. After the initial start, transistors 131 and 132 are provided a regenerative feedback current drive generated by windings 121 and 122 as explained later.
In the oscillation mode, transistors 131 and 132 are continuously turned on and off, so as to conduct current alternately through each of primary windings 111 and 112. The alternating current flow through the primary windings creates an AC voltage signal which is applied to a series combination of capacitors 162, 163 and lamps 181 and 182 coupled together in parallel. Capacitors 162 and 163 control the current flow through lamps 181 and 182.
A constant current flow network 154, comprising inductor 152, resistor 142 and diode 171, operates to maintain a substantially constant biasing current flow to the base terminals of transistors 131 and 132 respectively. The base-emitter junction of each transistor acts as a diode, and thus blocks any current flow from returning via windings 121 or 122 to drive terminal 102 through the transistors' base-emitter junction, provided that the voltage applied by the drive windings does not exceed the reverse base-emitter breakdown voltage of the transistors (as will be further discussed later). Diode 171 is configured so as to prevent the reverse flow of current in a direction from drive terminal 102 to constant current flow network 154.
The switching back and forth between transistor 131 and transistor 132 is enhanced by the regenerative feedback current from drive windings 121 and 122, and constant current flow network 154. As shown, windings 121 and 122 are disposed between drive terminal 102 and the base terminals of transistors 131 and 132, respectively. It is desirable to maximize the voltage level across drive windings 121 and 122, since a higher voltage level at the base terminals turns the transistors on and off more rapidly and more efficiently than a low voltage level, and allows a wider range of applied voltage.
As previously mentioned, the voltage at the base terminal of the transistors and across the windings increases and decreases in accordance with the circuit's oscillating nature, and can be represented by a corresponding sine-wave curve. Since transistors 131 and 132 are alternately being turned on and off, the base voltage of each transistor is 180 degrees out of phase with the other. Significantly, there exists a point within each half-cycle of operation of this circuit when the voltage signal of the base terminal of a transistor and the corresponding drive winding voltage passes through zero. This point occurs when one transistor is turning on while the other transistor is turning off. At this point, the switching action of the circuit may be interrupted because no current would be flowing to compel the corresponding transistor to turn on or off again. In order to prevent the interruption of the switching action and maintain a constant current flow to the drive windings and transistors, the circuit includes constant current flow network 154 previously described.
The voltages which can be utilized in this circuit are limited by the base-emitter breakdown voltage of the transistors, which is approximately 6.5 to 7 volts. This breakdown voltage limits the voltage level at drive terminal 102 to minus 3.5 volts. This follows because when one of the transistors, e.g.--131, is switched "on" its base-emitter junction acts like a diode to clamp the left-hand side voltage of drive winding 121 to a value near zero or to the common line negative voltage level of power supply 190. At the same time the voltage level at drive terminal 102 and the right-hand side of winding 122 and base terminal of transistor 132 is taken to a negative level by an amount that depends on the number of turns of winding 122, and hence the drive voltage of the windings. Thus, because of the limit imposed by the breakdown voltage of the transistors, the total voltage across windings 121 and 122 cannot exceed 7 volts. Hence only 3.5 volts will be generated at the center of the circuit. Exceeding the base-emitter breakdown voltage adversely affects the operation of the transistors and decreases the lifespan of the circuit.
Additionally, since the circuit must maintain a relatively small voltage between the base terminals of the transistors, the resistive value of resistor 142 of constant current flow network 154 is also required to be small. The current-defining resistor 142, in order to permit an appropriate current flow into the center of the circuit, must be in the range of 10 to 20 ohms. Since voltage and current are directly related, a small change in the input supply voltage causes the drive current to change significantly and the lamp to either go out, or to be over driven causing excessive loss. As such, this circuit is unsuitable for dimming applications, since the lamps can not be dimmed over a wide range.
Therefore, there exists a need for a parallel resonant ballast for a fluorescent lamp which permits fluorescent lamps to be efficiently dimmed over a wide range.